Validity and Reliability of Spinopelvic Parameters Measured on Computed Tomography

This study aimed to measure pelvic incidence (PI) and other spinopelvic sagittal parameters on supine computed tomography (CT) and to assess the validity and reliability of measurements from supine CT images when compared with standing x-ray images. Difficulties in superimposition of femur heads and obtaining the perfect midsagittal view of the sacral endplate may cause relatively low intra- and interobserver agreements. Some authors reported that PI values measured by CT had higher reliability, but both validity and reliability of CT measurement of spinopelvic parameters compared with standing x-ray imaging methods have not been reported previously. PI, pelvic tilt (PT), and sacral slope (SS) were measured on standing lateral x-ray and spinopelvic supine CT images of 33 asymptomatic volunteers. The mean PI, PT, and SS in standing x-ray images were 45.2°, 10°, and 35.3°, respectively, whereas those in supine CT images were 44.5°, 8.2°, and 36.2°, respectively. No significant differences were found in each parameter. Excellent correlations were found between each parameter obtained from x-ray and CT images. Intra- and interobserver reliabilities were excellent in both x-ray and CT image measurements, although those from CT images were higher. Spinopelvic sagittal parameters could be measured on supine CT by using a simple method with high reliability and validity; thus, CT could be a good alternative to standing x-ray imaging. In the supine position, PI does not change but PT decreases by a small amount and SS increases almost by the same amount because of the mathematical relationship between PT and SS (PI = PT + SS). Supine CT is an efficient diagnostic tool for the reliable extraction of spinopelvic sagittal parameters.